This invention relates to airport landing and takeoff systems and more particularly to a radar-optical transponding system suitable for use at airports and heliports.
In the prior art the instrument landing system (ILS) has been extensively used for landing aircraft at airports during low ceilings and low visibility weather conditions. ILS operates at very high frequencies (VHF) and ultra high frequencies (UHF). At these wavelengths antenna patterns are very broad and course stability is vulnerable to ground moisture content and spurious reflections from hangers or rising terrain near the approach paths. Further, the ILS localizer (azimuth determining beams) and glide slope (descent path) antenna systems may require special tailoring for use at most airports to avoid interference patterns and achieve acceptable course accuracy and stability. Not infrequently expensive grading is required for ILS site preparation. In addition to the airport ILS localizer and glide path antenna systems and equipment installations, the system requires three VHF ground marker beacons some distance (up to five miles) from the airport runway threshold.
A landing system employing microwave frequencies can be designed to have highly directional and narrow beams which can essentially eliminate spurious reflections from terrain or buildings, multipath, and sensitivity to ground moisture content etc. MLS because of its size and expense will be available only to major airports.
Additional information which can be supplied by a landing system (not the present ILS) is wind shear information, ground speed, range to runway, and distance to end of runway. The Federal Aviation Administration has determined that a comparison of ground speed to air speed during landing and takeoff is a way to detect (sense) and avoid wind shear. One accurate way to determine ground speed is to equip each airplane with an inertia navigation system; however, for many aircraft the expense involved for this approach is prohibitive. Another way which employed an airborne weather radar to range track a modulating reflector on the ground, has been tried. The problem with the radar/reflector approach lies in obtaining sufficiently high signal to ground clutter ratio, initial target (reflector) acquisition by the radar, and the modifications to the aircrafts weather radar to give it a range angle/track capability.
However, a modulating reflector (Luneberg lens), was developed and patented (U.S. Pat. No. 3,295,137, issued Dec. 23, 1966) in an attempt to provide such a target. The Luneberg lens was equipped as a passive, non-radiating, modulating radar reflector which produces a known amplitude modulated (AM) frequency which can be identified and tracked by the airborne radar. All other targets are ignored as long as the signal to clutter ratio is reasonable.
A problem with the Luneberg lens type modulating reflector, which will be described in pertinent part, hereinafter, is that it had to be mechanically spun. The reliability of mechanical spinning devices is subject to the environmental conditions. Further, no ideal material exists for construction of the lens. A foam plastic material was used, but the material severely limited its maximum modulation frequency, because for a 30" diameter Luneberg lens, the lens self destructed at about 600 revolutions per minute (RPM) and under 600 RPM the centrifugal force distorted the lens. Also, a problem associated with the Luneberg lens is achieving a lens radar cross section large enough to stand out above the local ground clutter present in a radar return signal Particularly, where it is desired to use the reflector on an offshore oil rig, ship, or other heliport locations.
Another problem with the airborne radar and ground reflector system lies in the high power radar transmitter (5 kw to 50 kw) required to provide an effective range of from 10 to 25 miles. To provide such power, microwave vacuum tube devices such as magnetrons, are generally used and the reliability of the modified magnetron type radar system is considered inadequate for a landing system. The addition of a redundant transmitter to improve system reliability is objectionable not only because of cost but also because of the additional power, space required, and the added weight.